Diabetes and obesity continue to represent major challenges to the health and health care systems of the USA and other countries. Current therapies are helpful, especially for type 2 diabetes, but they are still inadequate to prevent the negative effects of the Metabolic Syndrome on the cardiovascular system, cancer and other disorders. It has long been recognized that exercise is an excellent first line therapy for both diabetes and obesity [1]; however, many patients are unable to exercise sufficiently for a variety of reasons. A huge challenge has been to capture some of the benefits of exercise in a manner that can be useful medically for the very broad range of diseases for which exercise appears to provide benefit. Previous work on this grant has shown that the transcriptional coactivator PGC1? is induced with exercise in muscle and gives muscle many of the attributes seen in this tissue with chronic exercise. We show here that PGC1? expression in cultured muscle cells and transgenic mice stimulates the secretion of an activity that can drive the expression of UCP1 and other genes of the brown fat program in white adipose cells. Using global gene expression analyses, coupled with bioinformatics tools, we identify a key secreted protein as Fndc5, a muscle protein previously identified as an intracellular factor. We show here that Fndc 5 is cleaved and secreted as a novel 112 amino acid polypeptide termed irisin. Circulating irisin is increased with exercise in rodents and humans, and mildly elevated irisin in mice stimulates a browning of the white fat, with increased energy expenditure and improved glucose homeostasis. Aim 1 of this new application proposes purifying and cloning the cellular irisin receptor using an active fusion protein between irisin and the Fc moiety of human IgG. Alternatively, this tool can also be used to isolate the irisin receptor by expression cloning. Critical tests of the function of the irisin receptor will involve genetic manipulations in both cultured cells and in the adipose tissues of mice. Aim 2 examines the mechanism by which PGC1? controls irisin gene expression. We will employ a cultured muscle cell system to examine the chromatin marks at and near the Fndc5 locus to identify the key transcription factors through which PGC1? activates Fndc5 expression. We will also study whether manipulation of these key PGC1?-docking factors can regulate irisin expression in vivo. Post-translation modifications (PTM) of PGC1? have been shown to be important modulators of its function but there have been no systematic examination of PTMs in any biological system. In Aim 3 we will purify PGC1? from skeletal muscle under sedentary and exercised conditions. In collaboration with Steven Gygi and Pere Puigserver, we will determine all covalent modifications by quantitative Mass Spectrometry. Mutant alleles will be utilized both in vitro and in vivo to ascertain the physiological significance of these PTMs. Together these studies should lend important insights into the basic science of muscle and exercise as well as open new avenues for the rapid development of new therapeutics.